The present invention relates to low leakage solid state switches and, in particular, to solid state switches used in range-changing networks in electrical measurement devices.
Referring to FIG. 1, a typical range-changing circuit for a high impedance and/or low current, current measurement device is generally indicated by the numeral 10.
The noninverting input of the operational amplifier 12 is connected to a reference potential for the measurement (i.e., the measurements on a device under test are made with respect to V.sub.F). The inverting input of the amplifier 12 is connected to a test point on the device under test. In order to appropriately scale the input current from the test point, a range-changing network is provided between the output of the amplifier 12 and the inverting input of the amplifier 12. The range-changing network includes a series of parallel branches, each including a switch and a range impedance. For example, the switches 14, 16, 18 are each located in a range-changing network branch having a range impedance 20, 22, 24, respectively. The switches 14, 16, 18 may be, for example, reed relays. The range impedances 20, 22, 24 may be, for example, 2 G.OMEGA., 20 G.OMEGA., 200 G.OMEGA., respectively. To select a range, one of the switches 14, 16, 18 is turned "on" and the rest turned "off".
It should also be noted that the feedback from the output of the amplifier 12 through the range-changing network forces the inverting input of the amplifier 12 to V.sub.F.
Ideally, the switches 14, 16, 18 have infinite impedance when they are "off" and zero impedance when they are "on". It is possible to closely approximate this ideal behavior with reed relays.
However, reed relays are expensive, take up substantial space and provide a source of mechanical failures. Solid state switches can be smaller, cheaper and more reliable. Unfortunately, prior art solid state switches tend to be leaky when they are "off". As a result, they have not been suitable where measurements on the order of 10 picoamperes or less are desired.